1. Field of the Invention
The present invention relates to a buffered optical fibre and a method for improving the lifetime thereof.
More particularly, the present invention relates to a buffered optical fibre comprising a tight-buffer coating and a method for improving the lifetime thereof under high power and small diameter bend improving the energy removal therefrom.
2. Description of Related Art
An optical fibre generally comprises a core surrounded by a cladding (hereinafter both collectively referred to as “optical waveguide”), said core and cladding being preferably made of glass, and at least one exterior coating.
In many instances, the exterior coatings are two. The exterior coating directly contacting the optical waveguide is called “first coating” or “primary coating” and the exterior coating overlying the first one is called “second coating” or “secondary coating”. Typically, said first and second coatings are made of polymer material.
Certain applications require the optical fibre to be further coated by a buffer coating provided over the at least one exterior coating.
Typically, a buffered optical fibre can be used as semi-finished component to form a cable in association with other components as required by the use.
In some applications, the buffered optical fibre can be used as such to act as a cable. Examples of these applications are indoor and premises applications, cable termination, pigtails, patchcords and, more generally, those applications implying the optical fibre to be subjected to additional mechanical and friction stress often repeated in time.
When said buffer coating is provided in contact with the at least one exterior coating it is said “tight buffer”, when said buffer coating is in form of a tube having an internal diameter larger than the overall external coating diameter it is said “loose buffer”.
Sikora et al. (“Reduction in fibre reliability due to high optical power”, Electronics Letters, 10 Jul. 2003, vol. 39, No. 14) disclose that high power, 180° bend tests in samples of singlemode optical fibre show that some fibres can suffer catastrophic damage at optical power as low as 0.5 W at bend diameters of 13 mm. Damage at fibre bends is driven by an increase in temperature when power lost at the bend is absorbed by the coating.
The phenomenon caused, e.g., by the accidental bending of a fibre in loops with small radius, or by an excess of power transmitted can be also referred to as “thermal runaway”. Part of the guided light beam escapes from the fibre core and crosses the protective layers, where it is at least partially absorbed, and the absorption process heats the coating materials.
US 2004/0175086 teaches that a portion of the input light energy can be incident on the core/cladding interface at an angle less than the critical angle of incidence. Upon such an occurrence, this light energy passes from the core and continues through the interface between the cladding and the coating, because the conventional coating has a higher index of refraction than that of the cladding. This light energy may be absorbed by the coating or any surrounding materials and converted into heat energy. The heat energy can cause localized damage to the optical fibre and surrounding materials, which significantly reduces the operational life of the fibre. This is particularly consequential in high-power applications, such as but not limited to those where the transmission signal has a power above 0.5 W. Also, a severe bend, such as one having a radius or kink smaller than about 10 mm, may cause signal energy propagating along the core to be injected into the cladding. Again, the escaping light energy is converted to heat upon leaving the cladding, which can overheat a localized portion of the optical transmission fibre, resulting in premature failure.
US 2004/0175086 suggests to solve this problem with an optical transmission fibre, comprising:    a core having a first index of refraction;    a cladding material located around said core and having a second index of refraction less than said first index of refraction;    a first coating material located around a first portion of said cladding material and having a third index of refraction greater than the second index of refraction; and    a second coating material located around a second portion of said cladding material and having a fourth index of refraction less than said second index of refraction.
The Applicant observes that a drawback of this fibre is the need of carefully selecting refraction indexes of the coating layers sharply limits the choice of the first and second coating materials. Furthermore, this fibre has also the drawback that it cannot be “upjacketed” with a tight-buffer coating which would hinder dissipation of the light energy escaping the second coating material.
WO 2004/066007 teaches an optical fibre that can stabilize and propagate high-power light without causing damage and the like to the optical fibre even if the optical fibre is temporarily bent with small curvature diameters, through the use of coating material with little absorption of the escaped light in the optical fibre, in particular through the use of transparent UV-cured resin as the coating material. Alternatively, the coating layer of the optical fibre is formed by a primary coating layer made of ultraviolet-cured resin, secondary coating layer, and colored layer where the colored layer intermittently does not coat the secondary coating layer at a portion in the direction of the circumference.
The Applicant observes that also this fibre has the drawback that it cannot be “upjacketed” with a tight-buffer coating which would hinder dissipation of the light energy escaping the second coating material.
I. A. Davies et al. (“Optical fibres resilient to failure in bending under high power”, ECOC 2005 Proceedings, Vol. 3, 471-472) propose a reduced refractive index acrylate inner primary coating for overcoming the failure mode due to simultaneous high power and tight bending. However, although the greatest resistance to this failure mode is provided by the coating with lowest refractive index, the Authors suggest that an intermediate index may be practically advantageous considering the protection of networks elements downstream from the bend.
The Applicant thus noticed that there remains a need for an optical fibre “upjacketed” with a tight-buffer coating not hindering dissipation of the light energy escaping the second coating material and/or dissipation of heat energy derived therefrom.
US 2003/0133679 relates to an optical fibre including a glass or plastic core (or waveguide), a cladding on the waveguide, a primary coating on the cladding and a secondary coating on the primary coating. The optical fibre is coated with a flame retardant tight-buffer coating composition. The tight-buffer coating can be halogen-free or substantially halogen-free. Halogen-free flame retardants that have been found to be useful are flame retardant plasticizers and flame retardant acrylate oligomers.
U.S. Pat. No. 6,215,931 relates to flexible thermoplastic polyolefin elastomers for buffering a telecommunication cable element. Said thermoplastic polyolefin material may also contain organic or inorganic fillers. A reduction in the density of the material is advantageous because it allows for a reduction in cable weight. The buffer material is halogen free and can be made flame retardant.
The Applicant faced the problem of protecting the optical fibres from damages arising from heat originated inside the fibre by light transported therethrough. In particular, the Applicant faced the problem of providing this kind of protection by means of a solution applicable to an optical fibre without of changing the chemical or physical features of the at least one exterior coating or coatings and/or of the cladding layer of the fibre.
The Applicant perceived that the function of protecting the fibre from the said internally generated heat could be performed by a buffer coating made of a material endowed with specific physical properties.